Since their first introduction over 30 years ago, Ziegler-type catalysts have been widely used in the chemical industry for the production of olefin polymers, principally the production of homopolymers and copolymers derived from ethylene or propylene.
The early Ziegler catalysts are described in U.S. Pat. No. 3,113,115 to Ziegler et al. They may be characterized basically as a chemical complex derived from a halide of a transition metal from Groups IV-VIII of the Periodic Table, for example, titanium, vanadium, chromium, and zirconium and a metal hydride or metal alkyl based upon a metal from Groups I-III, most notably aluminum based compounds such as trialkyl aluminum and dialkyl aluminum halides. Also employed in Ziegler catalyst systems, particularly where it is desirable to achieve a highly stereospecific isotactic polymer are Lewis bases, or electron donors, commonly referred to as ED compounds. Numerous ED compounds are known in the art and include ethers, esters, and polysilanes or siloxanes.
Examples of Ziegler catalyst systems which are known in the art and to which this invention pertains include those disclosed in U.S. Pat. Nos. 3,793,294 (Luciani et al); 4,107,413 (Giannini et al); 4,187,196 (Giannini et al); 4,226,741 (Luciani et al); 4,294,721 (Cecchin et al); 4,390,454 (Cuffiani et al); 4,439,540 (Cecchin et al); 4,495,338 (Mayr et al); and 4,547,476 (Terano et al).
It is conventional practice to prepolymerize a Ziegler-type catalyst prior to introducing the catalyst into the polymerization reaction zone. The prepolymerization typically takes place in an initial, relatively small reactor prior to introduction of the catalyst into the main polymerization reactor. For example, U.S. Pat. No. 4,316,966 to Mineshima et al discloses a process in which a Ziegler-type catalyst is prepolymerized in an initial separate reaction zone with ethylene, propylene or another alpha-olefin depending upon the desired polymer product. The product contained in the prepolymerization step is recovered from the initial reaction zone and used in subsequent reaction zones. Other procedures for pretreating Ziegler-type catalysts under prepolymerization conditions are disclosed in U.S. Pat. Nos. 3,893,989 (Leicht et al), 4,200,717 (Abe et al) and 4,287,328 (Kikuta et al) and in British Patent Specification No. 1,580,635 (Mitsui).
The prepolymerization step conducted in accordance with the prior art teachings proceeds at a slow pace. A common expedient is to prepolymerize the catalyst in a batch-type operation in which the reaction zone is charged with the components of the catalyst system and the propylene or other olefin monomer then added over a number of hours while the mixture is agitated. An alternative procedure as disclosed in the aforementioned patent to Mineshima et al is to carry out the prepolymerization reaction continuously and couple it with the subsequent polymerization steps. Here, the residence time of the catalyst system in the prepolymerization reactor, while sometimes shorter than as in the case of the batch-type operation, is still a relatively long one.